Biology : Semester II


IntroductionSection 1 | Section 2 | Section 3 | Section 4 | Section 5

  Section Two:

Part 1 | Part 2 | Part 3 | Part 4 | Part 5 | Part 6 | Part 7

Biology : Birth of a Planet and Establishment of Life : Part One

The Precambrian:
Birth of a Planet and Establishment of Life

Earth is the only planet we know of that has life. Mars, our neighbor in the Solar System, is grudgingly revealing tantalizing clues about its distant past, including hints that life there may have been possible. The earliest part of the geologic time scale is the Precambrian, spanning some 2.5 billion years. Many major events occurred during this interval: the first cells, development of photosynthesis, the first eukaryotes, etc.

presentation View this presentation to learn more about the Precambrian Period. Be sure your volume is set at a reasonable level.

Origin of the Earth
Scientific estimates place the origin of the Universe at between 10 and 20 billion years ago. The currently most accepted theory is the Big Bang Theory, the idea that all matter in the Universe existed in a cosmic egg (smaller than the size of a modern hydrogen atom) that exploded, forming the Universe. Recent discoveries from the Space Telescope and other devices suggest this theory may need some modification. Evidence for the Big Bang includes:

  1. The Red Shift: when stars/galaxies are moving away from us the energy they emit is shifted to the red side of the visible-light spectrum. Those moving towards us are shifted to the violet side. This shift is an example of the Doppler effect. Similar effects are observed when listening to a train whistle-- it will sound higher (shorter wavelengths) approaching and lower (longer wavelengths) as it moves away. Likewise red wavelengths are longer than violet ones. Most galaxies appear to be moving away from ours.

  2. Background radiation: two Bell Labs scientists discovered that in interstellar space there is a slight background radiation, thought to be the residual remnant of the Big Bang.

Soon after the Big Bang the major forces (such as gravity, weak nuclear force, strong nuclear force, etc.) differentiated. While in the cosmic egg, scientists think that matter and energy as we understand them did not exist, but rather they formed soon after the bang. After 10 million to 1 billion years the universe became clumpy, with matter beginning to accumulate into solar systems. One of those solar systems, ours, began to form approximately 5 billion years ago, with a large "protostar" (that became our Sun) in the center. The planets were in orbits some distance from the star, their increasing gravitational fields sweeping stray debris into larger and larger planetesimals that eventually formed planets.

The processes of radioactive decay and heat generated by the impact of planetesimals heated the Earth, which then began to differentiate into a cooled outer crust (composed of the relatively light elements silicon, oxygen, and aluminum) and increasingly hotter inner areas (composed of the heavier and denser elements such as iron and nickel). Impacting bodies, such as asteroids, comets, and even small planetismals combined with beginnings of volcanism to release water vapor, carbon dioxide, methane, ammonia and other gases into the early Earth’s developing atmosphere. Sometime soon after this, life on earth began.

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